1. Field of the Invention
The present invention relates to a power transmission unit, and more particularly, the present invention relates to a one-way power transmission unit, and a method, for transmitting only one of a forward driving force and a backward driving force from a driving part to a driven part, a fusing unit driving apparatus for a duplex printer using the same, and a method for driving a fusing unit.
2. Description of the Related Art
In a mechanism that transmits forward and backward driving forces from one driving part to at least two driven parts, if one driven part requires the forward driving force but the other driven part does not require it, the forward driving force has to be prevented from being transmitted to the other driven part.
A printing apparatus such as a laser beam printer generally uses the same motor to drive a fusing unit, a paper discharging unit, and a duplex unit. However, it is impossible for the fusing unit to rotate in a backward direction (a paper discharging direction is defined as a forward direction), and thus, when the paper discharging unit rotates in the backward direction to invert a paper sheet, the printing apparatus requires an additional device to prevent the backward driving force from being transmitted to the fusing unit.
A dedicated one-way power transmission unit has been provided to prevent the backward driving force from being transmitted from the motor to the fusing unit, and it is illustrated in FIG. 1 by way of an example.
Referring to FIG. 1, a general one-way power transmission unit 1 comprises a driving member 10, a driven member 20, and a latch member 30.
A boss 11 is disposed at a center portion of the driving member 10, and a first latch gear 12 is formed around the boss 11.
Another boss 21 is disposed at a center portion of the driven member 20. The driven member 20 is also provided with a pair of interference portions (not shown) formed at both sides of another boss 21.
The latch member 30 is provided with a second latch gear 31 corresponding to the first latch gear 12 of the driving member 10, and a pair of protrusions 32 and 32′ to be located between the pair of interference portions of the driven member 20. The latch member 30 has a pair of inclined portions 33 and 33′ disposed between the pair of protrusions 32 and 32′.
The latch member 30 is assembled with the boss 11 of the driving member 10 in a movable manner along the length of the boss 11. The latch member 30 is also assembled with the driven member 20 by the protrusions 32 and 32′ and the interference portions.
When the driving member 10 rotates in a forward direction, the second latch gear 31 of the latch member 30 and the first latch gear 12 of the driving member 10 are engaged with each other, and higher portions of the inclined portions 33 and 33′ of the latch member 30 are brought into contact with a front end of another boss 21. Accordingly, the latch member 30 and the driven member 20 rotate in the forward direction in relation to the rotation of the driving member 10.
When the driving member 10 rotates in a backward direction, the latch member 30 is slid from the engagement of the first and the second latch gears 12 and 31. As a result, the power is prevented from being transmitted to the driven member 20. At this time, in the sliding of the latch member 30 from the driving member 10, lower portions of the inclined portions 33 and 33′ are brought into contact with the front end of another boss 21 of the driven member 20.
If the driving member 10 rotates back to the forward direction, the latch member 30 slightly rotates in the same direction as the driving member 10 and is moved toward the driven member 10 by the inter-operation between the inclined portions 33 and 33′ of the latch member 30 and the front end of another boss 21 of the driven member 20 so that the first latch gear 12 and the second latch gear 31 are engaged with each other. As a result, the latch member 30 and the driven member 20 rotate in the forward direction together with the driving member 10.
As described above, assembling of the latch member 30 with the boss 11 of the driving member 10 is an important factor in the one-way power transmission unit 1 to connect and disconnect the driving force efficiently. That is, in order to achieve the efficient connection and disconnection of the driving force of the one-way power transmission unit, the latch member 30 has to be assembled with the boss 11 of the driving member 10 in a smoothly movable manner along the length of the boss 11, and there has to be a predetermined level of friction between the latch member 30 and the boss 11.
However, it is difficult to set an assembly gap between the latch member 30 and the boss 11 of the driving member 10 sufficient to satisfy the above assembly conditions. If the latch member 30 and the driving member 10 do not have precise sizes to satisfy the above conditions, it causes defect in connecting and disconnecting the driving force.
Especially, if the inner surface of the latch member 30 and the outer surface of the boss 11 of the driving member 10 are lubricated by a lubricating oil or the like, when the driving member 10 rotates back to the forward direction, the latch member 30 does not follow the rotation of the driving member 10 immediately and is delayed in rotating in the forward direction. Therefore, there is an irregular time interval in switching from a disconnection state of the driving force to a connection state.
For example, when the general one-way power transmission unit is applied to the fusing unit driving apparatus of the duplex printer, the delay in re-driving the fusing unit causes malfunctions such as a paper jam and a paper crumple.